In the graphic arts, it is desirable to produce a four or more color proof to assist a color etcher in correcting a set of color prints prior to using them to produce color plates and also to reproduce the color quality that will be obtained during the printing process. The proof must be a consistent duplication of the half tone, and should neither gain nor lose color. Visual examination of a color proof should show the following characteristics:
1. Defects on the negative.
2. The best color rendition to be expected from press printing of the material.
3. The correct gradation of all colors and whether grays are neutral.
4. The need, if any, for subduing one of the colors and/or giving directions for altering the film negatives before making the printing plates.
A method of transferring colored images has been applied to the preparation of a color proofing sheet in multicolor printing. Thus, color proofing sheets, for multi-colored printing, have heretofore been made by using a printing press or a proof press while taking all the steps necessary for actual multicolor printing, but such a conventional method of color proofing has been costly and time consuming.
Photographic processes have also been used, especially photographic processes using photopolymers. There are two known types of photographic color proofing methods; namely, the surprint type and the overlay type.
In the overlay type of color proofing method, an independent transparent plastic support is used for producing a print of each color separation film by applying a photosensitive solution of the corresponding color, and a plurality of such supports carrying prints of corresponding colors are then superposed upon each other on a white sheet to produce a color proofing sheet. The overlay type of color proofing method has the disadvantage that the superposed plastic supports tend to darken the color proofing sheet, and, as a result, the impression of the color proofing sheet thus prepared becomes vastly different from copies actually obtained by a conventional printing press or a proof press. Its primary advantage is that it is quick and can serve as a progressive proof by combining any two or three colors in register.
In the known surprint type of color proofing method, a color proofing sheet is prepared by successively producing prints of different colors from different color separation films, respectively, by utilizing a single opaque support by applying toners, or by applying photosensitive solutions or coatings of photopolymers of corresponding colors on the opaque support in succession. Examples of some of these approaches are: U.S. Pat. No. 3,671,236 (317)-Transfer color key and U.S. Pat. No. 3,136,637 (3M)-Color key. An advantage of the surprint type of color proof is that the color saturation is not influenced by the plastic support. This method more closely resembles the actual printing process and eliminates the color distortion inherent in the overlay system.
Various processes for producing copies of an image embodying photopolymerization and thermal transfer techniques are known. See U.S. Pat. Nos. 3,060,023; 3,060,024; 3,060,025; 3,481,736; and 3,607,264. In these processes, a photopolymerizable layer coated on a suitable support is exposed, imagewise, to a process transparency. The surface of the exposed layer is then pressed into contact with the image receptive surface of a separate element and at least one of the said elements is heated to a temperature above the transfer temperature of the unexposed portions of the layer. The two elements are then separated, whereby the thermally transferrable, unexposed, image areas of the composite transfer to the image receptive element. If the element is not precolored, the tacky unexposed image may now be selectively colored with a desired toner. The colored matter adheres, preferentially, to the clear unpolymerized material. The lamination, exposure, and development steps are carried out for the respective colors, in sequence, thus making these processes time consuming.
U.S. Pat. No. 3,574,049 (1971), issued to Trentesaux-Toulemonde, provides a transfer process, wherein it is stated:
"The present invention provides a transfer process for printing a design on a final support which comprises (a) printing a design onto a temporary support, (b) super-imposing the temporary support and the final support, (c) applying heat and/or pressure to the super-imposed structure formed in (b), and (d) separating the temporary support from the final support which retains the printed design, the affinity of the mater of the design for the temporary support being lower than its affinity for the final support."
"The temporary support may comprise a coating, called a transfer coating, of a material for which the material of the design has an affinity lower than its affinity for the final support, and preferably the difference between the two affinities also increases with increasing temperature."
In the claims, the temporary support is coated on its face with a transfer coating (a release layer) of ethylcellulose which bears a pigmented film of vinylchloride polymers or copolymers. The teachings of the above patent allow only (a) a single transfer which is not feasible for multiple transfer, (b) transfer of vinylchloride polymer systems only are possible, and (c) the transfer is made possibe by a thermally sensitive release layer present between the image and the support. Futhermore, this process is not photographic, but relies upon impression printing. Finally, the polyvinylchloride printed image is transferred only to metallic aluminum. No reference is made to transfer papers or plastics.
In U.S. Pat. No. 3,721,557, issued to Process Shizai Co., Ltd., Tokyo, Japan, a method of transferring colored images is claimed which provides a stripping layer coated between the photosensitive element and the support. When the photosensitive layer is exposed to actinic light and developed, the more soluble portions are selectively removed to produce a visible image. The image-carrying support is pressed against a suitable adhesive coated receptor member and, subsequently, the carrier support sheet is stripped to accomplish the transfer of the image. A fresh layer of adhesive is applied to the receptor for each subsequent transfer. The two extra steps (1) of coating a stripping layer between the photosensitive coating and the support, and (2) application of adhesive between each image, are time-consuming and costly. In addition, the image build-up in a four-color proof is so thick that it gives rise to moire patterns and secondly the image is so distorted that the proof cannot be any cleaner or truer than an overlay proof.
Most of the cited processes are positive working systems. Some require costly and sophisticated transfer equipment and depend on sequential lamination (pressure and heat), exposure, and transfer (or delamination of the polymeric element) processes for each color used. This, of course, makes the total process very time consuming. The use of addition polymerizable elements for the production of relief images was first revealed by Gates (British Pat. No. 566,795--1945) but this invention was not found to be practical. Photopolymer elements of a more practical and preferred type relating to this invention are disclosed in Plambeck, U.S. Pat. Nos. 2,760,863 (1956); 2,791,504 (1957); 3,218,167 (1965); 3,458,311 (1969); Giangualano et al, U.S. Pat. No. 3,376,139 (1968); and Eastman Kodak Company U.S. Pat. Nos. 2,697,039 (1954); 3,497,356 (1970); and 3,519,424 (1970).
Finally, U.S. Pat. No. 3,775,113, issued to Bonham et al, covers a process for transferring positive images to receptor sheets. It should be noted here, that this process is for positive-working systems rather than negative-working systems and the photosensitive materials utilized are photolyzable rather than light-hardenable. In fact, it is stated in the patent that "A further objection to processes utilizing transfer sheets having a free radical photopolymerizable layer is that such layers are susceptible to oxygen inhibition and must be protected by a cover sheet or protective layer."
The distinguishing features of our invention, as will be shown in more detail below in the description of the invention, however, are that the unexposed negative-working, photosensitive layer is not thermally transferrable and the exposed and developed negative-working, photosensitive layer is thermally transferrable without the aid of a stripping layer between the negative-working, photosensitive layer and the carrier sheet.
One objective of this invention is to utilize a truly negative-working system in which color separation negatives are used for the exposure step and the light-exposed images obtained are composed of light hardened matter.
A further objective, when using photopolymerizable layers, is to expose the light sensitive coatings through the transparent polyester support in such a way as to produce a thin layer of crosslinked coating next to the support sheet and overlaid with enough uncrosslinked coating, or other oxygen impervious coating, so that polymerization of the light sensitive coating may proceed.
Another objective of this invention is to provide a new and practical process for transferring multicolored photopolymerized images from transparent overlay foils to a receptor sheet, using but one layer of adhesive, regardless of the number of colored images so transferred.
A still further objective of this invention is to carry out the transfer process, under pin register, and at a low uniform pressure and temperature so that high resolution of the image is obtained.
Another objective is to permit the use of any water resistant sheet or foil as a suitable carrier overlay, wherein the proper image adhesion is obtained both before and after development.
A further objective is to provide a suitable adhesive transfer process so that even porous paper may be used for the receptor sheet without starving the surface for adhesive.
Still further objects will be apparent from the description of the invention.